The X-ray beam detector has an arrangement of light-sensitive detector elements, and there is a scintillator layer on this arrangement. The object of the scintillator material is to convert individual X-ray quanta into a plurality of light quanta, and these light quanta can then be registered with the aid of the light-sensitive detector elements (photodetectors). The arrangement of the light-sensitive detector elements conventionally forms a two-dimensional grid and, in order to define a respective X-ray image, a grayscale value, emerging from a measurement with the aid of the detector element, is defined for each detector element. The X-ray image is formed by the grayscale values in a pixel arrangement that corresponds to the arrangement of the detector elements.
The scintillator now emits light in a plurality of directions, and said light may then also be scattered in certain circumstances. An impact point of a light quantum on the scintillator therefore corresponds to various impact points of the light quanta produced by the X-ray quantum. Hence, the actual X-ray image may be slightly “smeared”. The contrast is reduced.
It is for this reason that the scintillator layer is not allowed to be too thick so that it can generate sufficiently high-contrast X-ray images. Then again, the number of X-ray quanta being converted into light increases as the scintillator layer becomes thicker.
The scintillator layer was previously produced from cesium iodide (CsI). By way of example, in the case of a detector for X-ray angiography, the scintillator layer made of cesium iodide has a thickness of 500 μm at a density of approximately 3.6 g/cm3. If a voltage of 125 kV is applied to the X-ray beam tube, and if an object made of water (“patient”) to be imaged has a thickness of 400 mm, the maximum absorption is only approximately 40%. Hence, 60% of the X-ray radiation is lost.
Nevertheless, cesium iodide has previously been considered the gold standard because it is possible to form rod-shaped crystals made of this material: provision is made for a multiplicity of needles, which run parallel with respect to one another. At the walls of these needles or rods, there is total internal reflection of the light generated therein, and so the light is routed to the detector elements in more or less a straight line, as in an optical fiber.
X-ray beam detectors with a scintillator layer made of cesium iodide are very expensive, for example compared to gadolinium oxysulfide, Gd2O2S, the otherwise also possible material.
Thus, conventional X-ray beam detector have the property that they are expensive, but nevertheless only register a comparatively low proportion of the X-ray radiation.